Photodynamic therapy (PDT) is one of a novel modalities which has recently been exploited to eradicate various microorganisms. In our study we have evaluated bactericidal efficacy of PDT in the presence of 5-δ aminolaevulinic acid (ALA). Porphyromonas gingivalis were incubated with increasing concentration of ALA and subsequently irradiated by progressive light doses. Complete killing effect was obtained for bacteria irradiated with 25J/cm2 in ALA solution final concentration of 1mM, 5mM, 10mM. Statistical analysis has revealed ALA concentration to be a major factor responsible for eradication of bacteria. The latter may be attributable to the known ALA dark toxicity.